Voltage controlled oscillators (VCOs) are useful in a wide variety of electrical circuits. For example, VCOs are building blocks for phase locked loops (PLLs) which are used in FM receivers and in other communications systems. A VCO receives an input voltage, and generates an output signal at a frequency proportional to the input voltage.
Several types of VCOs are known in the art. One type is a crystal-controlled VCO. The crystal VCO includes an inverter connected across the terminals of a crystal. One capacitor has a first terminal connected to a first terminal of the crystal and another terminal connected to a second terminal of the crystal and another terminal connected to a negative power supply voltage. A varactor is connected between the second terminal of the first capacitor and the negative power supply voltage.
Another type is a modification of a ring oscillator. A ring oscillator has an odd number of inversion stages. The output of the last stage is connected to the input of the first stage. Oscillation occurs because a given logic state on the input of a first inversion stage propagates through an odd number of inversion stages. A signal inverted from the original input of the first stage eventually is presented to the input of the first stage. The output of the last stage thus oscillates at a frequency determined by the propagation delay through all the stages. A ring oscillator may be made into a VCO by making the delay proportional to the input voltage. For example, if complementary-metal-oxide-semiconductor (CMOS) technology is used, each inverter stage includes a standard CMOS inverter. The ring oscillator VCO includes an additional transistor in the CMOS inverter. A P- or N-channel transistor is connected between the source of the transistor of the same conductivity type and the source-side supply. The input voltage is then applied to the gate of the additional transistor, thus controlling the switching speed.
Yet another known type of oscillator is a relaxation oscillator. The relaxation oscillator relies on charging times of a capacitor to determine the frequency of the output signal. As the input voltage varies, the rate of charging of the capacitor similarly varies.
Each of these known oscillators has problems in their operation which become more critical as integrated circuit performance increases and cost decreases. The ring and relaxation oscillator VCOs are each susceptible to noise-induced jitter on the output signal. The crystal VCO requires both an external crystal and varactor, which are expensive in terms of both component cost and pin count. Thus, new VCO designs with intolerance to noise-induced jitter and low cost are needed.